Composition of Pure Substances - AP Chem Unit 1, Topic 3

In this video, Mr. Krug discusses how to determine the elemental composition of pure substances and how to calculate the empirical formula of a substance from mass percent data. He also shows the difference between empirical formula and molecular formula, working several examples.

[@Name-sb2in]

Thank you so much, this was very easy to understand! My teacher went over this today and was completely lost earlier tbh lol

[@JeremyKrug]

I’m so glad my video was easy to understand! Best wishes on your course this year, you will do great!

[@ulixes327]

Thank you, this video was very helpful

[@JeremyKrug]

You're very welcome. Thanks for watching!

[@haneenalali7486]

this video is great thank you

[@JeremyKrug]

Thanks for watching, best wishes with chemistry this year!

[@mathiseasy9005]

Thank you so much sir, I had a question. How do you know which atom comes first? For example in the last example involving the compound of nitrogen and oxygen, why would you put nitrogen atoms before the oxygen atoms while writing the empirical formula? Thanks again!

[@JeremyKrug]

Usually we place the more electronegative atom last in a binary compound. So since F is more electronegative than N, we'd write NF3. However, there are a few exceptions for no really good reason, thus we have NH3, even though N is more electronegative.

[@maxpercer7119]

if it is determined the molecular mass is 194.0 amu for caffeine - how is that determined ?

[@JeremyKrug]

This could be determined using some form of instrumental analysis, most likely a mass spectrometer or other instrument. There are also other more 'simple' lab methods that we could use to determine molecular mass / molar mass of a substance; for example, for a gas we could use gas density.